Narrative:

I was working sector 25 radar position at ZDV. Aircraft X checked on at FL330 inbound to apa. Shortly after his check-on, I issued a clearance to aircraft X to 'descend and maintain flight level two seven zero (FL270).' the pilot queried me as to whether this was a pilot's discretion descent or not. I told the pilot that I needed him to descend for traffic. The pilot replied: 'ok, we're out of three, three, zero for seventeen.' I did not catch this errant readback. About the time aircraft X departed FL270 in a descent, aircraft Y was at or less than 5 mi away at FL260 on a converging course. The conflict alert activated and I instructed aircraft X to maintain FL270 and turn right to a heading of zero nine zero (090 degrees) immediately. I instructed aircraft Y to turn left heading zero one zero (010 degrees). The closest proximity was 4 mi/400 ft. There were 2 radar hits with less than standard separation (5 mi/1000 ft) before separation was established and maintained again.

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Original NASA ASRS Text

Title: ZDV RADAR CTLR MISSED INCORRECT READBACK CAUSING A C441 TO DSND TO ALT OCCUPIED BY A WW24 IN CRUISE FLT.

Narrative: I WAS WORKING SECTOR 25 RADAR POS AT ZDV. ACFT X CHKED ON AT FL330 INBOUND TO APA. SHORTLY AFTER HIS CHK-ON, I ISSUED A CLRNC TO ACFT X TO 'DSND AND MAINTAIN FLT LEVEL TWO SEVEN ZERO (FL270).' THE PLT QUERIED ME AS TO WHETHER THIS WAS A PLT'S DISCRETION DSCNT OR NOT. I TOLD THE PLT THAT I NEEDED HIM TO DSND FOR TFC. THE PLT REPLIED: 'OK, WE'RE OUT OF THREE, THREE, ZERO FOR SEVENTEEN.' I DID NOT CATCH THIS ERRANT READBACK. ABOUT THE TIME ACFT X DEPARTED FL270 IN A DSCNT, ACFT Y WAS AT OR LESS THAN 5 MI AWAY AT FL260 ON A CONVERGING COURSE. THE CONFLICT ALERT ACTIVATED AND I INSTRUCTED ACFT X TO MAINTAIN FL270 AND TURN R TO A HEADING OF ZERO NINE ZERO (090 DEGS) IMMEDIATELY. I INSTRUCTED ACFT Y TO TURN L HEADING ZERO ONE ZERO (010 DEGS). THE CLOSEST PROX WAS 4 MI/400 FT. THERE WERE 2 RADAR HITS WITH LTSS (5 MI/1000 FT) BEFORE SEPARATION WAS ESTABLISHED AND MAINTAINED AGAIN.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.